To insert a line break in an HTML document, you can use the <br> element. The <br> element is a self-closing tag, which means it does not have a closing tag. It simply inserts a line break wherever it appears in the document.

Here is an example of how to use the <br> element:

<p>This is a paragraph.<br>This is a new line.</p>

The output of this code would be:

This is a paragraph.
This is a new line.

To insert a horizontal line in an HTML document, you can use the <hr> element. The <hr> element is also a self-closing tag. It inserts a horizontal line (also known as a horizontal rule) wherever it appears in the document.

Here is an example of how to use the <hr> element:

<p>This is a paragraph.</p>
<hr>
<p>This is a new paragraph.</p>

The output of this code would be :

This is a paragraph.
----------------------------------
This is a new paragraph. 

Note that the <br> element and the <hr> element do not have any attributes, so you cannot customize their appearance or behavior. If you want more control over the appearance and behavior of lines and breaks in your HTML document, you can use CSS (Cascading Style Sheets).